68 Bulletin of the Brooklyn Entonnological Society Vol. XV 



a positive rheotropism, but this habit is by no means the mani- 

 festation of a fixed and mechanical reflex, since on occasion they 

 turn and with the same faciHty move down the stream or across 

 it, in pursuit o'f prey or away from enemies, or with purposes 

 less clear to the observer. 



Among these water-striders may be found most striking ex- 

 amples of pterygo-polymorphism, or diverse wing development 

 among individuals of a single species. Gerris canformis is 

 always macropterous in this region, while a near relative, G. 

 remigis Say, also found- as a rule on running water, is usually 

 but not always apterous. Of G. marginatus Say, generally 

 macropterous, more rarely apterous, there are often found 

 examples having wings of one half the normal length, as well as 

 various intermediate conditions, and in G. canaliculatus Say 

 both macropterous and apterous forms occur frequently, the one 

 sometimes developing from eggs produced by the other, as de 

 la Torre Bueno has shown.^ M. hesperius, T. pictus, R. rileyi, 

 and R. obesa are usually found in the apterous condition in the 

 north, but winged individuals of all these species may be occa- 

 sionally met with. Among thousands of examples of R. rileyi 

 recently examined at Cold Spring Harbor, L. I., but one was 

 macropterous, the rest quite apterous. Renter^ and de la Torre 

 Bueno* have offered speculations, notably lacking in experimental 

 evidence, on the subject, but an adequate explanation of the phe- 

 nomenon is yet to be made and the problem is certain to repay 

 investigation by modern methods, since even the genetic behavior 

 of the forms, quite possibly Mendelian, is almost entirely un- 

 known. These insects, especially the species of Gerris, can be 

 bred successfully in captivity by following the methods developed 

 by de la Torre Bueno. ^ 



The occurrence here of the species enumerated above is of 

 sorne distributional interest, since none of them, excepting Gerris 



2 Op. cit., p. 248. 



3 Polymorphisme des Hemipteres. Bull. Soc. Ent. France, 1875, pp. 225- 

 236. 



*Life history and habits of the margined water strider, Gerris margin- 

 atus Say. Ent. News, Vol. 28, 1917, p. 297. 



*" Notes on collecting, preserving and rearing aquatic Hemiptera. Can. 

 Ent., Vol. 37, 1905, pp. 137-142. 



